Electrostatic image reproduction with pre-exposure

ABSTRACT

A METHOD FOR FORMING A LATENT ELECTROSTATIC IMAGE ON A PHOTOCONDUCTIVE PLATE WHEREIN THE PLATE IS UNIFORMLY PREEXPOSED TO ELECTROMAGNETIC RADIATION HAVING A LOW INTENSITY SUCH THAT THE CHARGE ACCEPTANCE OF THE PLATE TO AN APPLIED ELECTROSTATIC CHARGE IS AT LEAST ABOUT 80% OF ITS CHARGE ACCEPTANCE IF THE PLATE WERE NOT SO PRE-EXPOSED. PREFERABLY SUCH PRE-EXPOSURE SHOULD BE USED IN CONJUCTION WITH AN IMAGE EXPOSURE IN WHICH SUBSTANTIALLY ALL THE LIGHT STRUCK AREAS OF THE PLATE RETAIN A MEASURABLE AMOUNT OF CHARGE.

Jan. 26, '1971 I 1.. A. CARLSON 3,553,307 ELECTROSTATIC IMAGEREPRODUCTION WITH FEE-EXPOSURE Filed 001:,7. 1966 3 Sheets-Sheet 1 6, L.A. CARLSON T 3,553,307 I ELECTROSTATIC IMAGE REPRODUCTION WITHPIKE-EXPOSURE Filed Oct-7, 1966 v s Sheets-Sheet z 2.5 3 [XPOSl/Rf 77[56(0/VD5) Jan. 26,1971 L. A. CARLSON' 3,558,307 A ELECTROSTATIC IMAGEREPRODUCTION WITH PIKE-EXPOSURE" Filed 0on7} 19 6' I 1 s Sheets-Sheet 5Q Q Q o' Q Q Q a s (5110A) yrs'v United States Patent Olfice 3,558,307Patented Jan. 26, 1971 3,558,307 ELECTROSTATIC IMAGE REPRODUCTION WITHPRE-EXPOSURE Lee A. Carlson, Ashland, Mass., assignor to DennisonManufacturing Company, Framingham, Mass., a corporation of Nevada FiledOct. 7, 1966, Ser. No. 585,108 Int. Cl. G03g 13/22 US. Cl. 961 ClaimsABSTRACT OF THE DISCLOSURE A method for forming a latent electrostaticimage on a photoconductive plate wherein the plate is uniformlypreexposed to electromagnetic radiation having a low intensity such thatthe charge acceptance of the plate to an applied electrostatic charge isat least about 80% of its charge acceptance if the plate were not sopro-exposed. Preferably such pre-exposure should be used in conjunctionwith an image exposure in which substantially all the light struck areasof the plate retain a measurable amount of charge.

This invention relates to xerography or electrophotography and moreparticularly to improvements in image reproduction, especially images oflow contrast.

Electrophotography as normally practiced includes the steps of providinga uniform electrostatic charge to the surface of an electrophotographicplate, exposing the thus charged plate to a pattern of light and shadowto discharge light struck areas and produce a latent electrostaticimage, and development of the latent image to a visible image. The plateusually comprises a photoconductive insulating layer, such as seleniumor a zinc oxide binder layer on a relatively conductive backing such asmetal or paper. Common forms of electrophotographic apparatus provide arelatively movable path between the plate and stations for performingthe operating steps, a plate moveable past fixed stations being mostcommon although apparatus providing a fixed plate and sequentiallymoveable stations is known.

In the above image reproduction, difiiculties have arisen inreproduction of continuous tone, faint originals such as pencildocuments, large image areas, and in reproducing originals having avariety of indicia, sometimes in various compositions, colors andintensities. One exposure for such originals often fails adequately toreproduce all such indicia.

It is accordingly the principal object of the present invention toprovide improved xerographic image reproduction, especially in solidimage areas, low contrast originals, and from originals having varyingindicia in color, composition and intensity, especially spirit, diazoand NCR copy, all with less critical exposure limits.

Pre-exposure of xerographic plates comprising photoconductors such asZinc oxide to light to which the photoconductor is sensitive has beenstudied. Generally, preexposure is considered undesirable because itdecreases charge acceptance and contrast and decreases the .rate oflight decay, although US. Pat. No. 3,249,430 appears to suggestpre-exposure to high intensity radiation as a means of controllingcontrast.

In accordance with the present invention, it has been found thatnotwithstanding its disadvantages, pro-exposure of xerographic plates touniform illumination of low intensity improves image rendition oforiginals, preferably when combined with under-exposure of the plate,after electrostatic charging, to an image pattern of light and shadow.Thus, in one aspect, the invention comprises the xerographic methodproviding a photosensitive xerographic plate having a photosensitivesurface comprising a photoconductor, uniformly pre-exposing thephotosensitive surface to electro-magnetic radiation which can beabsorbed by said photoconductor and to which said surface is sensitive,providing a substantially uniform electrostatic charge on said surface,and, preferably, underexposing said charged surface to a pattern oflight and shadow to produce a latent electrostatic image in whichsubstantially all light struck areas retain measurable apparent surfacevoltage charge. Preferably the photoconductor is dye-sensitized zincoxide, the exposures are to visible, infra-red and/or ultraviolet light,the under-exposure comprises no more than the foot candle secondsnecessary fully to discharge non-image background areas, and is morepreferably about thereof, and the pre-exposure to about 5 times, in footcandle seconds, the under-exposure.

In a further aspect, the present invention comprises a apparatus forpre-exposing a xerographic plate uniformly to radiation comprising alamp in a housing attached to a corona charging device relativelymoveable with respect to said plate into position to provide anelectrostatic charge thereon, said housing having a slit directingradiation from said lamp onto said plate prior to relative movement intosaid position. Preferably the lamp is an incandescent tungsten lamp andis disposed to provide about 60 foot candle seconds of pre-exposureradiation on the xerographic plate.

The present invention can be better understood by reference to theaccompanying drawings in which:

FIG. 1 is a plan view with parts broken away, of a corona chargingdevice with attached pre-exposure lamp and housing;

FIG. 2 is a section on the line 2-2 of FIG. 1;

FIG. 3 is a plot of apparent surface voltage (A.S.V.) in volts on axerographic plate versus time of exposure to light passing throughfilters of varying density, but without use of pre-exposure; and

FIG. 4 is a plot of A.S.V. versus time of exposure through the samefilters but after uniform pre-exposure of the xerographic plates.

Referring to the drawings, FIGS. 1 and 2 illustrate a corona chargingdevice for a flexible xerographic plate to which a pre-exposure lamp andhousing are attached. The corona device itself is more fully describedin copending application of Mie'kka and Smith, entitled Corona ChargingDevice, and filed on about even date herewith. Generally it comprisessubstantially identical front and back halves 11 and 12 releasablyjoined by means of electrically conductive tab and lock means 13. Eachhalf comprises corona wires, half 12 having corona wires 15 and 17 andhalf 11 having wires 16a and 17a, the wires being spring mounted toelectrical end leads (not shown) for attachment to a high voltagesource. The wires are shielded by conductive shields 25 and 25a to whichconductive baffles 24 and 24a are conductively attached as by welding orthe like. Across the open face of shields 25 and 25a lacings of spaceddielectric guide strands 30 and 31 are employed as more fully disclosedand claimed in the above mentioned copending applicatlon.

Halves -11 and 12 define a slot 14 through the corona device for axerographic plate such as zinc oxide-binder coated paper which movesthrough the device along a path 15 in the direction indicated by thearrow in FIG. 2.

Assembled to half 11 is a housing for a pre-exposure lamp 51 terminatingin socket means 52 for connection to a source of electrical power.Housing 50 is attached to half 11 by means of tabs 53-53 which receivelocking detents 54-54 on shield 25a. Halves 11 and 12 and housing 50 arealso assembled by means of end plates 55-55.

In operation, the xerographic plate such as paper coated with particlesof dye-sensitized photoconductive zinc oxide dispersed in an insulatingbinder is passed along path 15 with the coating side facing half 11 andpre-exposure lamp 51. Lamp 51 is turned on for a time sufiicient topreexpose the plate coating to the desired intensity just prior to entryinto slot 14 as shown by ray arrows in FIG. 2, exposure being throughslot 56 in housing 50, slot 56 being, for example, of a width toexposure a transverse section of the plate surface about one inch inwidth.

As indicated above, it is known that high intensity preexposure ofdark-adapted photoconductors such as zinc oxide to radiation which thephotoconductor can absorb and to which the plate is sensitive fatiguesthe photoconductor so as to decrease its charge acceptance and its rateof light decay, the decreases generally becoming greater with increasingintensities and duration of pre-exposure. Pre-illumination useful hereinis directed onto the photosensitive surface and is of low intensity,preferably an intensity by which the initial charge acceptance of theplate, at constant corona, is at least 80% and preferably at least 90%of the initial charge acceptance without preexposure. Most preferably,the pre-exposure is about 5 times the subsequent image exposure to lightand shadow in the xerographic process, for example 60 foot candleseconds of pre-exposure for about 12 foot candle seconds of imageexposure. Thus an illustrative process comprises exposing azinc-oxide-binder photoconductive layer to 60 foot candles ofpre-illumination from lamps 51, thereafter charging the layer by meansof corona wires 16-1611 and 17-17a to a negative potential of about 500volts, and subsequently illuminating the thus charged layer to a patternof light and shadow of maximum intensity of about 12 foot candleseconds, all as more fully explained hereinafter. The thus formed latentelectrostatic image can be developed or otherwise used by conventionalxerographic techniques such as treatment with liquid toner or the like.As is also well known, the image exposure can be formed in any suitableway, for example by passing the illumination through a transparency,reflecting it from an opaque original or the like.

The image exposure according to this invention is preferably anunder-exposure. Normally, it is desired that the charged xerographicplate receive sufiicient illumination in background, non-image areas tosubstantially reduce the electrostatic charge at such areas to zero,thus to provide least background development and greatest imagecontrast. However, to obtain the advantage of better image resolution,especially from weak or low contrast originals, it has been found that,with pre-illumination, exposures to lesser illuminations are normallynecessary.

FIG. 3 of the attached drawings illustrates a series of curves showingapparent surface voltages (A.S.V.) in volts on a zinc oxide-bindercoated paper zerographic plate versus time of exposure to about 12 footcandle seconds of illumination, each curve representing the light decaywhen exposed from a white original through no filter curve) or throughneutral filters of various density indicated by the numbers 0, .1, .2,1.0 beside each curve, 0 representing 100% reflectance, 0.1 about 80%,0.2 about 63%, 0.3 about 50%, etc. From the curves it can be seen that,with an initial A.S.V. of greater than 500 volts, the 0 curve declinedto zero in about -6 seconds, the higher density curves retaining greatervoltages for longer times. However, it will be noted that the lowdensity cunves 0.1, 0.2, etc., corresponding to weak originals, decayalmost as fast so that little developable charge remains when backgroundin the 0 curve is carried to zero. Since the curves are so closelyspaced, underexposure would not materially increase contrast even ifbackground development were tolerated. Thus low contrast gray originalsare poorly reproduced.

In the measurements shown in FIGS. 3 and 4, A.S.V. was measured by knownapparatus and techniques dis- 4 closed in an article by E. C. Giaimo inR.C.A. Review, vol. 22, No. 4, pages 780-790.

FIG. 4 illustrates data similar to FIG. 3 but wherein the plates of eachcurve were each pre-exposed to 60 foot candle seconds from a showcase,8T, 40 watt, 120 volt tungsten filament bulb giving illumination in boththe visible and infrared wavelengths disposed about 2% inches from thephotosensitive plate moving along path 15 (FIG. 2) at a rate of about 4/2 feet per second. As the curves show, light decay is decreased so thatabout 9 seconds are now required to discharge the 0 curve toapproximately zero. However, low density filter curves 0.1, 0.2, etc.are spaced apart further than in FIG. 3. Thus the charge difference, forexample, between curves 0 and 0.1 at 7 seconds is much greater than at 9seconds so that under-exposure produces a greater contrast although alsoproducing some greater background. It has also been discovered, however,that pre-exposure produces more uniform images in large areas such asbackground (in a black and white original, for example) so that theslight greying of the background obtained in prints with under-exposureis actually often less visually noticeable than with less grey and lessuniform prints without either pre-exposure or underexposure. Further,large image areas appear to be more uniformly developed, improvinglegibility even though contrast may actually be decreased. Moreover, insome document reproduction, some increase in background may be tolerableto obtain better legibility of weak images. The more uniform imageresolutions in larger areas may be due to a more equalized surfacepotential obtained in the corona, after pre-exposure.

As indicated above and by reference to FIG. 4, about or more preferablyof the exposure necessary to reduce the zero curve to zero potential ispreferred as under-exposure for optimum resolution of light gray toneswith minimum tolerable background print development.

It has been thus discovered that the combination of pre-exposure, atrelatively low intensities which do not decrease charge acceptance involts more than about 20 percent and more preferably not more than about10%, at a constant corona exposure, and preferably with underexposure,produces advantages which often outweigh the known disadvantages ofpre-exposure alone. This is especially true in increasing the range ofreproduction of varying original indicia at a given exposure.

The effect on charging rate and acceptance of increasing pre-exposuretime to a foot candle tungsten source on a zinc-oxide-binder paperxerographic plate is shown in Table'I on next page.

TABLE 1 Charging Charge Light rate, volts/ acceptance, decayPre-exposure time, sec. sec. volts time see.

None (control) 610 550 7 575 520 7 575 520 8 555 500 9. 5 500 500 10 480480 10 Preexposure was found to follow the reciprocity law, at leastwithin the range of 10 to 100 foot candles, the effect of 6.0 seconds at10 foot candles being substantially the same as 0.6 second at 100 footcandles.

It should be understood that the foregoing description is for thepurpose of illustration and that the invention includes all equivalentsfalling within the scope of the appended claims.

What is claimed is:

1. The xerographic method of forming a latent electrostatic image on aphotoconductive xerographic plate with improved rendition of lowcontrast images comprising (a) providing a photosensitive xerographicplate having a photosensitive surface comprising a photoconductor,

(b) uniformly pre-exposing the photosensitive surface to electromagneticradiation which can be absorbed by said photoconductor the intensity ofsaid radiation being such that the charge acceptance of said plate to auniformly applied electrostatic charge is less than, but at least about80% of, the charge acceptance of said plate without said uniformpre-exposure,

(c) providing a substantially uniform electrostatic charge on saidsurface substantially immediately following said pre-exposure, and

(d) exposing said charged surface of a pattern of light and shadow toproduce a latent electrostatic image in which substantially all lightstruck areas retain measurable apparent surface voltage charge.

2. The method according to claim 1 wherein said preexposure is to anintensity such that said charge acceptance is less than, but at leastabout 90% of, the charge acceptance of said plate without said uniformpre-exposure.

3. The method according to claim 1 wherein said photo-- conductor isparticulate zinc oxide dispersed in an insulating binder.

4. The method according to claim 3 wherein said preexposure and saidimage-exposure are to visible or ultraviolet light.

5. The method according to claim 3 wherein the time duration of saidimage-exposure is no more than about of the time required, at constantintensity, to reduce the apparent surface voltage of fully light struckareas substantially to zero.

6. The method according to claim 3 wherein the time duration of saidimage-exposure is no more than about of the time required, at constantintensity, to reduce the apparent surface voltage of fully light struckareas substantially to zero.

7. The method according to claim 5 wherein said preexposure is aboutfive times said image-exposure when both are measured in foot candleseconds.

8. The method according to claim 7 wherein said preexposure is aboutfoot candle seconds and said imageexposure is to about 12 foot candleseconds.

9. The xerographic method of forming a latent electrostatic image on aphotoconductive xerographic plate with improved rendition of lowcontrast images comprising (a) providing a photosensitive xerographicplate having a photosensitive surface comprising a photoconductor,

(b) uniformly pre-exposing the photosensitive surface to electromagneticradiation which can be absorbed by said photoconductor the intensity ofsaid radiation being such that the charge acceptance of said plate to auniformly applied electrostatic charge is less than, but at least aboutof, the charge acceptance of said plate without said uniformpreexposure,

(c) providing a substantially uniform electrostatic charge on saidsurface substantially immediately following said pre-exposure, and

(d) exposing said charged surface to a pattern of light and shadow toproduce a latent electrostatic image.

10. The method according to claim 9 wherein said photoconductor isdye-sensitized zinc oxide.

References Cited UNITED STATES PATENTS 2,638,416 5/1953 Walkup et a196-117.5X 3,052,540 9/1962 Greig 96-1.7 3,103,445 9/1963 Bogdonofi et a1117-17.5 2,863,767 12/1958 Vyverberg 96-1 2,979,403 4/ 1961 Giaimo 9613,041,167 6/1962 Blakney et a1. 96-1.4 3,249,430 5/1966 Metcalfe et a1.96-1.3 3,268,331 8/1966 Harper 961 3,355,289 11/1967 Hall et a1. 961.4

CHARLES E. VAN HORN, Primary Examiner U.S. Cl. X.R.

